running low speed - air in pump basket

[drband]

Hi: Converted my Superpump 1hp to a 1hp 2 speed this spring... works great! My question is... why am I getting a bit of an air bubble in my pump basket when I run it on low speed? The air bubble seems to grow a little bit if I run it continuously for a day or so. Water seems to be moving fine, no air coming out of returns, and there is never any air in the pump basket when I run it on full speed. Just trying to figure out if there is some kind of problem here or not. Thanks.

[cleancloths]

If it is not causing a problem - like air in the returns, or cavitation, it is probably not a problem. I have a Pentair Intelliflow pump that is continuously adjustable. When I run it on a very low speed I typically have about 2" of air sitting in the top of the pump basket, and the pump runs fine. I can get rid of that by turning the pump up hight to fill the basket and then slowing it down, but it does not cause any problems in operation.

[drband]

Thanks. Sounds the same as mine. I've just gotten the 2-speed and this was new to me.

[AnnaK]

I have the very same issue with my Pentair 2-speed.

It normally runs on low for several hours during the night and after a few days the air bubble in the pump pot will have grown from the size of a penny to maybe 3" in diameter. When I shut the pump off there'll be some air bubble coming up through the skimmer basket.

If I set it to high speed it's somewhat turbulent in the pump pot for a few moments until the pot is completely filled and there's just the one small bubble on top.

No water leaks anywhere around the equipment. No air coming out the return. I've concluded it's some mysterious weirdness that I'm just not going to worry about.

[mas985]

I have the same thing going on but I believe that it is due to outgassing. The pump basket pressure is below atmospheric so any dissolved air will tend to outgass at a rate relative to the pressure difference. On high speed the flow rate is sufficient to expell any air bubbles that may form but on low speed the air tends to collect on the top of the pump basket albeit very slowly as in my case. I suspect those with waterfalls and spillovers experience this more than those with no way to increase the dissolved air in the water.

You can also see this occur when the pump is off but in the sun (or a sealed bottle in the sun). As the water heats up, it can hold less air and bubbles will start to form on the pump lid. I see this all the time in my spa pump when it is off.

However, if air collects quickly, it is likely a pump lid air leak as the lid does not seem to seal as well on low speed as it does on high speed when the suction is stronger.

[CarlD]

I have the same thing and never really worried about it. The pump works fine, in its 8th season.

[AnnaK]

That was a really nice explanation, Mark. Thank you very much!

[drband]

I have the same thing going on but I believe that it is due to outgassing. The pump basket pressure is below atmospheric so any dissolved air will tend to outgass at a rate relative to the pressure difference. On high speed the flow rate is sufficient to expell any air bubbles that may form but on low speed the air tends to collect on the top of the pump basket albeit very slowly as in my case. I suspect those with waterfalls and spillovers experience this more than those with no way to increase the dissolved air in the water.

You can also see this occur when the pump is off but in the sun (or a sealed bottle in the sun). As the water heats up, it can hold less air and bubbles will start to form on the pump lid. I see this all the time in my spa pump when it is off.

However, if air collects quickly, it is likely a pump lid air leak as the lid does not seem to seal as well on low speed as it does on high speed when the suction is stronger.

Thanks! That really sounds about right for what is happening. I also note that tonight, now that it's cooler, there's no bubble at all on low speed, so I think the temp is in play, as per Mark's explanation.